The real answer to your question "What should be the partition scheme for ubuntu 18.04 lts 500gb hdd?" is: how you wish it to be. There is no rules. You should partition according on how you want to organize your data.
By default, the installer will establish one single partition containing system and user data. For a personal computer (not a server), this is the simplest solution. The drawback is that if you do a clean install in the future, then you also will need to place your data back. This is the simplest configuration, and there is nothing wrong with that.
Some people prefer to have a separate home partition. The root partition can then be as small as 15 GB or less. The second partition is then mounted on /home. Thus, all user configuration data and user data will be on the other partition. The convenience here is that your user data can remain in case you do a fresh reinstall in the future.
None of these scenarioś eliminates the need to having good backups of your personal data on removable drives or media.